Selling on Amazon could require a GTIN exemption form.
The exemption via letter template will work if the SPOD merchant are planning to sell their products under the brand name "SPOD".
If the SPOD merchant uses/wants to sell his products under a different brand name, the exemption may not work.
To do this for a different brand name it would be best to register the brand on Amazon Brand Registry or registering a trademark off Amazon, and request a UPC/GTIN exemption which is usually granted without problems for private label brands.
Please find a GTIN exemption letter from SPOD below: